This proposal requests continued support for the genetic, serologic and structural dissection of the genes and gene products coded by the I region of the mouse major histocompatibility complex. These Ia antigens (DR in human) play a central role in immune regulation and thus are of major importance in human disease. During the next five years, emphasis will be placed in the following areas: 1) gene conversion or copy mechanism has been established as the major force through which genetic polymorphism occurs for MHC genes. We will screen for such occurrences for Ia genes by making specific matings, surveying sublines of major inbred strains of mice as well as utilizing crossreactions as indicators of gene conversion events. This would give us an insight into the frequency and mechanisms underlining this very important event. 2) Using random fragment linked polymorphisms with specific Ia gene probes, we will determine whether there is a 'hot spot' in the E beta gene involved in recombination events. Further, we will also determine the origin of the E alpha gene in several new recombinants. 3) Molecular heterogeneity of Ia molecules will be explored using several monoclonal antibodies in biochemical analysis. Our aims are to see whether there are additional Ia molecules present and whether they are a result of newly identified Ia genes or other mechanisms such as alternate splicing, and post-translational modifications. We will also determine the contribution of proteoglycans in generating Ia heterogeneity. 4) We will continue our studies on the expression of Ia antigens on T cell to determine why resting T lymphocytes do not express Ia antigens. We will look for soluble mediators which might turn on Ia expression during T cell activation, possible role of methylation and presence of tissue specific enhancer sequences. 5) We will initiate an extensive program to produce trransgenic mice for Ia antigens. We expect this to be a difficult but worthwhile project. 6) We will continue our serological and biochemical characterization of Ia antigenes by trying to identify Ia on subpopulations of cells; we will try to identify new Ia genes by reciprocal immunizations using new recombinant strains; and look for quantitative differences in expression of Ia on B cells by FACS analysis. 7) Even though not related to Ia, we will use our new L/D recombinant to characterize the D region genes, their orientation, boundaries and number of gene products. Finally, we will continue our service role of providing mouse strains and reagents to laboratories throughout the United States and other parts of the world.